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Exciton interstate dynamics in photoexcited quantum wells
manipulated by strong narrow-band THz pulses
Sangam Chatterjee
TAC-LSUM, 6.10.2013
Acknowledgements
Experiment:• Nils Rosemann, Alexey Chernikov, Benjamin Ewers, Martin Koch• Martin Teich, Sybine Zybell, Harald Schneider, Manfred Helm
(Helmholtz-Zentrum Dresden-Rossendorf)• William D. Rice, Junichiro Kono (Rice University)
Theory:• Andrea Klettke, Lukas Schneebeli, Mackillo Kira, Stephan W. Koch
Samples: • Galina Khitrova, Hyatt M. Gibbs (The University of Arizona)• Stephan Schartner, Gottfried Strasser (TU Wien)
2
Outline:
• Introduction: Excitons, Autler-Townes Effect & Ionization
• 1s-2p Interstate Coherence and 2s-2p Quantum Beats
• Population Control: Rabi-Flopping of 1s-2s Excitons
• Conclusions & Outlook
3
Introduction: From Atoms to Solids to Excitons…
crys
tal
bands4
3
2
n=1
ener
gy
atom
levels “atom-like dispersion”
exci
ton
+-
5
Time-Resolved Spectroscopy on Semiconductors
• Different physical phenomena different time scales
Optical Excitation Polarization and dephasing
Carier injection and cooling Formation many-body excitations
time0
Introduction: Optical vs. THz Spectroscopy
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Eg
Valence Band – “p-type”
k
E
THz Photon
Conduction Band – “s-type”
Detection of Excitons:Optical pump + THz probe
•
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Polarization Manipulation:Weak Optical + Strong THz Excitation
•
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Polarization Manipulation:Weak Optical + Strong THz Excitation (II)
• M. Wagner et al., Phys. Rev. Lett. 105, 167401 (2010)
• B. Ewers et al, Phys. Rev. B 85, 075307 (2012)
Reminder: Driven 2-Level-System (I)
• Linear absorption of a 2LS:
E0
Reminder : Driven 2-Level-System (II)
• Rabi-Splitting, Mollow triplet
E0
Reminder : Autler-Townes Effect
• Rabi-Splitting, Autler-Townes effect,
E0
Messages & Questions (I)
E0 E0
Inter-level Coherences:Optical-Pump – THz-Four-Wave Mixing
15
t
to Ge:Ga Detector
Optical-Pump – THz-Probe
1s exciton lifetime: • ~330ps (THz)• Similar to 1s TRPL lifetime
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Optical-Pump – THz-Pump – THz-Probe
1s exciton lifetime: • ~330ps (THz)• Similar to 1s TRPL lifetime
Opt. Pump – THz Pump – THz Probe• Interference pattern• “Shelving“ at later times
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Optical-Pump – THz-Pump – THz-Probe
1s exciton lifetime: • ~330ps (THz)• Similar to 1s TRPL lifetime
Opt. Pump – THz Pump – THz Probe• Interference pattern• “Shelving“ at later times
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Optical-Pump – THz-Pump – THz-Probe
1s exciton lifetime: • ~330ps (THz)• Similar to 1s TRPL lifetime
Opt. Pump – THz Pump – THz Probe• Interference pattern• “Shelving“ at later times• Oscillation period: 118µm
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Optical-Pump – THz-Pump – THz-Probe
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1s exciton lifetime: • ~330ps (THz)• Similar to 1s TRPL lifetime
Opt. Pump – THz Pump – THz Probe• Interference pattern• “Shelving“ at later times• Oscillation period: 118µm
Optical-Pump – THz-Four-Wave Mixing
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Decay ~50ps• Similar to 2s TRPL lifetime• Mechanism: acoustic phonons
Overlayed by slow oscillation• Period: 100ps• Energy: 40µeV• Quantum beat of 2s-2p
Splitting confirmed independently by magneto-PL
Next: Population Manipulation – Optical & THz Pumping, TRPL Detection
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Resonant with 1s
Resonant with 1s – 2p
FEL
Population Control:Optical & THz Pumping, TRPL Detection
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THz OFF THz ON
Energy E-E1s (meV)
Tim
e (p
s)
0
1
Population Control:Optical & THz Pumping, TRPL Detection
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Population Control:Optical & THz Pumping, TRPL Detection
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Theory:Coulomb-Induced Symmetry Breaking
2-particle exciton correlation
Exciton dynamics
Conservation law
Diffusive scattering
Coulomb scatteringbreaks THz-dipolesymmetry
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Messages & Questions (II)
• Excitonic Autler-Townes Effect• Multi-THz-Photon Ionization• Coulomb-Mediated Coupling
of 2s and 2p Exitons• Shelving of Excitons
(no carrier loss)
THz-Power-dependence?
30
Back to Population Manipulation:Optical & THz Pumping, TRPL Detection
31
Resonant with 1s
Resonant with 1s – 2p
FEL
Back to Population Manipulation:Optical & THz Pumping, TRPL Detection
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Resonant with 2s
Resonant with 1s – 2p
FEL
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Resonant 2s Optical Excitation
Resonant 2s Optical Excitation
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FEL off FEL on
Energy (meV)Energy (meV)
THz-Power-Dependence
Resonant 2s pumping:• Nonmonotoneous PL strength• Density-dependent period
(Coulomb-induced change of dipole-matrix element)
Resonant 1s pumping:• Similar behaviour• Less pronounced due to
„unaffected“ 1s emission
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Conclusions & Outlook
Manipulation of exciton populations using strong THz pulses• Unexpected strong 2s photoluminescence after 1s-2p excitation• Evidence of Coulomb-mediated coupling of 2s and 2p levels • „Shelfving“ of excitons in states with large angular momenta
– Phys. Rev. Lett. 110, 137404 (2013)
• 1s-2p transition predominantly homogeneously broadened• 1s-2p coherence times several 100’s of ps• Observation of quantum beat corresponding to 2s-2p separation• Secondary emission shows Rabi-Flopping between 1s and 2s
Outlook: Coherent State Control, THz Gain...
36
Messages & Questions (III)
• Excitonic Autler-Townes Effect• Multi-THz-Photon Ionization• Coulomb-Mediated Coupling of
2s and 2p Exitons• Shelving of Excitons
(no carrier loss)
• Interlevel Transitions:homogeneously Broadened
• Interlevel Interband Coherence• Dephasing and Decay by
Acoustic Phonons
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38
2s
pum
p1
s pu
mp
FEL off FEL on
Resonant 1s Optical Excitation
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FEL off FEL on
Energy (meV)Energy (meV)
Nonresonant Excitatation, Long Time Delay
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THz-Power-Dependence: 2s Optical Pumping
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THz-Power-Dependence
2s optical pumping 1s optical pumping
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Optical-Pump – THz-Four-Wave Mixing
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ΔtTHz-THz(ps) 2000-100 100
TH
z In
ten
sit
y (
arb
. U
nit
s)
Optical-Pump – THz-Four-Wave Mixing
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d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o
ΔtTHz-THz(ps) 2000-100 100
TH
z In
ten
sit
y (
arb
. U
nit
s)
Outline:
• Introduction: Excitons…
• Interband Polarization Control:– Excitonic Autler-Townes Effect and THz-Field ionization
• Population Control: Coupling of 1s-2s excitons– Excitation: Optical-Pump – THz-Pump – Detection: Time-Resolved Photoluminescence
• 1s-2p Interstate Coherence and 2s-2p Quantum Beats– Opt.-Pump - THz-Probe / Opt.-Pump - THz-Pump - THz-Probe – Degenerate Four-Wave Mixing
• Conclusions & Outlook
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• Show 1s-2s- coupling• Explain showing theory• Wonder about coherence of this transition• Shown FWM• FWM lifetime roughly equal to 2s PL lifetim• 1s-2p homogeneously broadened (PP-lifetime ~ FWM lifetime• Mechanism for both is AC phonon coupling• Beat show signatures of 2s-2p by beating• Energy separation independently veryfied by magnetp-PL
bhattacharya et al. In press• If coherent can we see rabi oscillations• Yes, we can in P Intensity of 1s and 2s pl varies with changing THz
fluence46
Experimental Setup
• Source: 1kHz Ti.Sapphire Amplifier120fs, 1mJ, 800nm pulses
• Coaxial geometry for NIR & THz
Strong Single-Cycle THz-PulsesSample: Narrow-Linewidth MQW
THz pulse
1s hh exciton @ 4K30x Ga0.96In0.04As, 8nm
Experiment: THz-Induced Bleaching
Experiment: Time Evolution
Experiment: Density Dependence
Experiment – Theory Comparison
Experiment Theory
Weak Optical + Strong THz Excitation (IV)
• B. Ewers et al, Phys. Rev. B 85, 075307 (2012)
Introduction: THz Experiments in Semiconductors
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